Current transplant management dampens the allogeneic immune response but does not deter eventual graft loss. This imposes limitations that compromise patient survival and rehabilitation outcomes. The compelling conclusion is that chronic immunosuppressive drug therapy is not a satisfactory long-term solution. Tolerance induction has potential to offer an exit from this stalemate. The goal of Project I is to implement successful translation of tolerance to human transplantation. A novel tolerance strategy developed in an established, rhesus macaque allotransplant model using day-of-transplant tolerance induction, one with potential for translation to cadaveric donor transplantation, will be examined. Preliminary data suggest the early posttransplant period circumscribes a permissive window in which functional tolerance is induced and remains stable for periods exceeding three years. The proposed studies are designed to extend these results and elucidate immunologic mechanisms responsible for induction and maintenance of kidney allograft tolerance in this model. Peritransplant treatment with a novel immunosuppressive drug combination, F(Ab)2- or sFv alpha-CD3 immunotoxin (IT) AND Deoxyspergualin (DSG) will be used to induce tolerance. A synergy between these agents establishes an immunologically quiescent milieu, characterized by transient physical absence of T cells and mature dendritic cells (DC) in lymphoid tissues. DSG has been shown to arrest NF-kappaB-dependent DC maturation in macaques. The hypothesis of the project is that early absence and slow recovery of mature DC in lymph nodes in pivotal in preventing DC-induced activation of new T cells during peripheral repopulation. To examine this hypothesis, the IT plus DSG treated recipients will be reconstituted with mature DC of donor or recipient origin within and subsequent to this period to determine if this intervention can precipitate rejection. On a parallel track, functional and phenotypic alterations in T cell memory after IT induced T cell-specific ablation will be examined. Finally, the specificity of tolerance and the role of persisting donor antigen in maintenance of the tolerant state will be investigated. In vitro and in vivo immunologic testing will be performed to validate tolerance. Since no measurable hematopoietic microchimerism in this tolerance model has been found, a determination will be made of whether the presence of 1 degree tolerant kidney graft antigens are required to maintain a tolerant state that permits acceptance of 2 degree donor kidney, and if so, how long this requirement persists. The proposed studies will generate original information relevant to tolerance in non-human primates, i.e., the role of DC maturation in induction of tolerance and the role of antigen persistence in maintenance of tolerance. Elucidation of these mechanisms should assist translation of tolerance to humans.